Instrument list: Full Complement of Particle and Field Instruments: High
Resolution Electron and Ion Detectors (UNH), VLF, ELF and DC Measurements
(0-20 kHz) made with the Electric Field Experiment (Cornell), Waveform
Snapshot Receiver, High Frequency Electric Field Instrument (Dartmouth)

Main Science

The Dartmouth high-frequency receiver on PHAZE II provided fully resolved
waveform measurements of the component of the electric field parallel to the
background magnetic field. This allowed auroral Langmuir waves and related
phenomena to be fully resolved.

The nature of the waves depends on the ratio of the plasma frequency to the
electron gyrofrequency. At high altitudes where the plasma frequency
is less than the gyrofrequency, called the underdense regime, the plasma
waves occur just below the plasma frequency and are organized into bands:
multiple narrowband structures which are constant in frequency and last
from a fraction of a second to tens of seconds. Sometimes these bands are
punctuated by an intense wave burst when their frequency matches the plasma
frequency, and sometimes no such burst occurs. We put forth that
these bands represent conversion of auroral Langmuir waves to whistler mode
in the inhomogeneous plasma. (The plasma frequency is the upper bound of
the whistler mode in the underdense plasma regime, in which these modes are
naturally connected.) Sometimes the causative Langmuir waves are
intercepted by the rocket and observed as a plasma frequency burst, and at
other times the rocket misses the causative Langmuir wave burst. As is well
known, auroral Langmuir waves are sporadic and bursty in nature, and hence
the mode-converted whistler waves just below the plasma frequency in the
underdense plasma consist of discrete bands rather than a continuous
emission. Modeling using the WHAMP code supports this hypothesis.See McAdams et al., 1999, for
details.

At low altitudes where the plasma frequency exceeds the gyrofrequency,
called the overdense regime, the plasma waves occur just above the plasma
frequency. They are also organized into narrow-band features, but these are
not constant in frequency but rather decrease in frequency with time. They
last typically for 100 ms or so. They come in pairs or sets of up to five
or six parallel emissions evenly spaced in frequency, changing in frequency
together. We call these wave structures "chirps." We put forth that
they arise from trapping of the nearly parallel (but slightly oblique)
Langmuir waves in field aligned density cavities. The Langmuir wavevector
is primarily parallel to the field and constant, so as to resonantly
interact with the auroral electron beam, but there is a small component of
the wavevector perpendicular to the magnetic field which can reflect from
the sides of the density cavity and interfere with itself. The density
cavity thereby impresses an eigenmode frequency structure on the trapped
Langmuir waves. Some fraction of the wave energy escapes and carries this
frequency structure with it as it propagates away, though its lifetime is
short due to heavy damping once the wavevector refracts in the
inhomogeneous plasma. Theoretical calculations support this hypothesis,
showing that the observed frequency spacing of the wave modes is
predicted by the observed density cavity size and depth, electron beam
velocity, and electron density. For more details about the
experimental data, see McAdams and LaBelle,1999. For more details about the
theory, see McAdams et al., 2000.

The high frequency experiment on PHAZE-II provided electron density
measurements from observations of wave cutoffs. Such measurements during
times of lower hybrid solitary structures confirmed that those structures
are associated with density depletions. Using wave cutoffs to infer the
density is not susceptible to effects of plasma inhomogeneity on collected
currents, unlike previous measurements using Langmuir probes, which were
controversial. For
details, see McAdams et al., 1998.

The high frequency receiver on PHAZE II also remotely sensed LF whistler
mode emissions well below the local gyrofrequency or plasma frequency,
apparently generated at greater altitudes in the aurora.
Surprisingly, these emissions are highly structured, consisting of multiple
discrete features rising or falling in frequency, not unlike chorus
observed at VLF associated with trapped electrons or fine structure of
auroral kilometric radiation. The frequency range, hundreds of kHz,
is comparable to AKR but much greater than VLF chorus. In some cases,
harmonic emissions were observed.For details, see LaBelle et al., 1999.

The high frequency receiver on PHAZE II detected numerous Langmuir wave
bursts at the local plasma frequency. Within each burst the electric field
amplitude is sampled numerous times. Statistics of the bursts reveal a
Gaussian dependence of probability of occurrence (logarized) versus square
of the electric field amplitude, but in more than half the cases, the
distribution switches to an E-2 power law at large
amplitudes. A Gaussian dependence is predicted by stochastic growth
theory. The power-law tails may indicate a nonlinear turbulent wave
dynamics. These data have been presented by Samara and LaBelle,
2001, and a paper is in progress.